This is the first application filed for the present invention.
The invention relates to the field of micro-imaging and, in particular, to methods and apparatus for acquiring high magnification images of a tilted and/or uneven sample using a micro-imaging system having a shallow depth-of-field.
In the field of micro-imaging a high magnification imaging instrument is used to acquire images of a sample. Magnification powers of 100xc3x97 or more are used for micro-imaging semiconductor integrated circuits (IC) to extract design and layout information for the purposes of design verification, product quality assurance and reverse engineering. At such high magnifications, a parameter of the high magnification microscope known in the art as the depth-of-field becomes very important.
According to what is know in the art as the xe2x80x9cThin Lens Approximationxe2x80x9d, while imaging an object using a theoretical optical system only an infinitesimally thin object plane in front of the optical system is in focus on an infinitesimally thin image plane, behind the optical system. The depth-of-field corresponds to the thickness of the image plane and therefore, in theory, the depth-of-field approximates zero. In practice, while imaging an object, features behind and in front of the theoretical object plane are also focused on the image plane. The depth-of-field is the thickness of the slice around the object plane that can be imaged in-focus.
Imaging objects at most one to two orders of magnitude smaller than constituent optical elements of an optical imaging system operating at small powers of magnification, the depth-of-field is large enough to capture an entirety of such objects in focus. However in micro-imaging sample IC""s using a 100xc3x97 power magnification: the optical elements are at most a few orders of magnitude larger than traces on a sample IC (xcx9c1 mm:xcx9c1 xcexcm). The width of traces on the sample IC is just wide enough that interference/diffraction effects are minimal when using ultraviolet light, and the pixel size of the CCD is comparable to the trace width. This results in a dept-of-field that is about the size of the width of a trace or about the thickness of a deposition layer on an IC.
Semiconductor components manufactured on a silicon substrate of an IC are several deposition layers in height. Traces interconnecting components on the silicon substrate transcend deposition layers. The components and interconnecting traces form a relief on the silicon substrate. Focusing not only becomes very important, autofocusing techniques are unsuitable because a range of focus settings of the micro-imaging system will appear to provide in-focus images, each image correctly focusing on different features distributed over a range of deposition layers.
U.S. Pat. No. 5,647,025 entitled xe2x80x9cAUTOMATIC FOCUSING OF BIOMEDICAL SPECIMENS APPARATUSxe2x80x9d which issued on Jul. 8, 1997 to Frost, et al. describes an apparatus for inspecting biological specimens and a method for automatically focusing on features using morphological criteria such as brightness, contrast, size, shape, texture and context. The apparatus is adapted to extract a focus measure concurrent with performing pattern recognition. The pattern recognition is optimized for biological cell detection in a particular size range and having a particular geometry. Methods for detecting biological cell nuclei are also presented. While this invention has merit, it is not suited for micro-imaging sample IC""s to extract design and layout information. The methods described by Frost only provide suitable autofocusing at 4xc3x97 magnification with a field of view of 1.4 mm square. At this magnification the depth-of-field has a substantial thickness enabling reliable focusing on discrete biological cells having cell nuclei. A comparable depth-of-field is not available at 100xc3x97 magnifications required for micro-imaging a sample IC.
Frost""s methods provide a timely inspection of a slide having approximately 700 fields of view. In micro-imaging sample IC""s a surface of interest is typically divided in excess of 10,000 fields of view each corresponding to a tile-image to be acquired. Autofocusing operations performed according to Frost""s teachings to acquire tile images would be time consuming and therefore unsuitable. Minimizing the time taken to acquire tile images is very important as pointed out in co-pending United States patent application entitled xe2x80x9cMETHOD AND SYSTEM FOR RECALIBRATION DURING MICRO-IMAGING TO DETERMINE THERMAL DRIFTxe2x80x9d, which was filed on Jun. 15, 2000 and assigned Ser. No. 09/594,169, the specification of which is incorporated herein by reference.
Therefore in micro-imaging a surface of interest for a sample IC, there is a need for methods and apparatus for providing focus settings to enable the acquisition of a very large number of tile-images of the surface of interest.
It is therefore an object of the invention to provide an interface for selecting locations for a plurality of focus points on a surface of interest of a sample IC to be micro-imaged and to select a focus setting associated with each focus point.
It is another object of the invention to compute a virtual focus surface that substantially mimics a surface topology found by components and interconnecting traces manufactured on sample IC silicon substrate, using a plurality of focus point locations and the associated focus settings.
It is yet another object of the invention to compute a tile image focus setting at a tile image focus location based on the focus surface and the plurality of focus points.
According to one aspect of the invention, there is provided an apparatus for micro-imaging an uneven surface of interest of a sample comprising means for selecting and storing positional coordinates of focus points associated with a sample coordinate space defined by the sample, to create a focus point list, means for determining an associated focus setting for each focus point in the focus point list, and means for generating a focus surface using the focus point list, as well as means for extracting a tile image focus setting at a tile image focus location from the focus surface, and means for positioning a micro-imaging system to acquire a tile image at the tile image focus setting.
The means for selecting and storing positional coordinates of focus points associated with a sample coordinate space preferably comprises a man-machine interface for sending control messages to the micro-imaging system, and receiving image data from the micro-imaging system. The means for determining a focus setting associated with each focus point further comprises means for extracting a focus measure from image data received from the micro-imaging system. The means for extracting a focus measure is prefreably an algorithm that performs pixel operations on the image data to extract, for example, a sharpness measure. The means for generating a focus surface using the focus point list comprises means for grouping focus points from the focus point list into focus point groups, the focus point groups being stored in a focus point group list. The means for grouping focus points into focus point groups comprises a focus point grouping algorithm, the positional coordinates and the associated focus setting of each focus point in a focus point group forming a focus facet, and the focus surface comprises abutting focus facets. The focus point grouping algorithm is preferably a mesh generation algorithm, for example, a triangular mesh generation algorithm. If so, each focus point group is a focus point triad. A preferred triangular mesh generation algorithm is the Delaunay triangulation algorithm, which is well known in the art. In order to ensure accurate focusing, the means for grouping focus points into focus point groups preferably further comprises a focus point group exclusion algorithm for excluding from the focus point group list a focus point group having substantially collinear focus points.
The means for extracting a tile image focus setting at a tile image focus location from the focus surface comprises an algorithm that selects a focus facet coincident with the tile image focus location, interpolates at the tile image focus location the focus settings associated with the focus points in the focus point group associated with the selected focus facet and sets the tile image focus setting to the interpolated focus setting. If the tile image focus location is not coincident with a focus facet, the means for extracting a tile image focus setting at a tile image focus location from the focus surface comprises an algorithm for selecting a closest focus point to the tile image focus location and setting the tile image focus setting to the focus setting associated with the closest focus point.
The invention also provides a method of generating a focus surface for determining a focus setting for a micro-imaging system used to capture micro-images of a surface of interest of a sample. The method comprises steps of: selecting a plurality of focus points respectively having focus point positional coordinates with respect to a sample coordinate space, to create a focus point list, at least some of the focus points being selected in close proximity to each other in the vicinity of an abrupt change in elevation of the surface of interest; determining a focus setting for each focus point; and compiling a list of focus point groups from the focus point list. The focus point list is stored after it is created.
Each of the focus point groups comprises three focus points that form a focus point triad. Each focus point triad defines a triangular mesh cell in the sample coordinate space. The combination of the focus point positions and the focus point settings of the focus points defining a triangular focus facet and the focus surface comprises adjacent triangular focus facets. Mesh cells are generated by a mesh generation algorithm using the plurality of focus points. The mesh generation algorithm preferably comprises a triangulation algorithm, such as the Delaunay triangulation algorithm.
The invention further provides a method of determining a tile image focus setting for micro-imaging a tile image constituent of an image-mosaic representative of a surface of interest of a sample having at least a one of a tilted and an uneven surface of interest. The method comprises steps of selecting a tile image to be acquired, the tile image having an associated tile image focus location with respect to a sample coordinate space defined by the sample; determining a focus facet coincident with the tile image focus location using a focus facet list associated with a focus surface of the sample, each focus facet in the focus facet list being defined by a focus point group, and focus points in each focus point group having respective positional coordinates with respect to the sample coordinate space, and an associated focus setting; and, interpolating the focus settings of the focus points of the focus facet coincident with the tile image focus location to determine a tile image focus setting for acquiring the tile image. The tile images have a predetermined shape and the tile image focus location is typically a geometrical center of the tile image.
The focus facet list is preferably parsed to exclude focus facets from the focus facet list that have substantially collinear focus points, in order to ensure that such focus facets are not used for focus setting. The focus facets are excluded from the focus facet list by computing a collinearity measure using positional coordinates of the focus points of the focus facet. The collinearity of the focus points of a focus facet is determined by, for example, computing an area of a mesh cell associated with the focus facet. The computed area is used as the collinearity measure. When a collinearity measure of the focus points is determined to be less than a predetermined threshold, the focus facet is excluded from the focus facet list.
For tile images having tile image focus locations outside all focus facets in the focus facet list, and tile images having tile images focus locations coincident with focus facets having substantially collinear focus points, the method preferably further comprises steps of selecting from among focus points in a focus point list, a closest focus point to the tile image focus location; and, assigning the focus setting associated with the closest focus point to the tile image focus setting.
The invention also provides a method of acquiring tile image constituents of an image-mosaic representative of a sample having at least a one of a tilted and an uneven surface of interest that is micro-imaged using a high magnification micro-imaging system. The method comprises steps of selecting a tile image to be acquired, the tile image having an associated tile image focus location with respect to a sample coordinate space defined by the sample; determining a focus facet coincident with the tile image focus location by parsing a focus facet list of focus facet constituents of a focus surface, each focus facet in the focus facet list being defined by a focus point group, and each focus point in the focus point group having positional coordinates with respect to the sample coordinate space, as well as an associated focus setting; interpolating the focus settings of the focus points of the focus facet coincident with the tile image focus location to determine a tile image focus setting; positioning the micro-imaging system in relation to the sample so as to acquire a tile image associated with the tile image focus location; adjusting a focusing mechanism of the micro-imaging system to the tile image focus setting; and acquiring the tile image.
A primary advantage of the invention is ensuring that the micro-imaging system focuses correctly on a selected sample IC, and in particular on features revealed at each deconstructive step in the process of reverse engineering the IC. A further advantage is the generation of a virtual focus surface that closely follows the relief of the surface of interest to provide tile image focus settings at tile image focus locations. This eliminates reliance on autofocus while acquiring tile images using the micro-imaging system, which is known to be unreliable for that purpose.